I don't think you lens choice is very good for paintings. As ahockley explained you need to avoid direct reflection. Which means light can not be in camera's "family of angle" (region of direct reflection). A wide lens has wide family of angle. A longer lens will be ideal in this case.
– VikasSep 12 '11 at 18:13

3 Answers
3

Lighting: You'll generally want even diffused light across the entire painting. Unless you have a bit of experience with flash, this is easier to set up with continuous lighting because you can see the light as you make adjustments.

Composition: Artwork is generally photographed head-on with the camera in line with the center of the piece.

White balance: as you note, setting the white balance for your shooting environment will ensure accurate representations. The "white round thing" you note is usually called a white balance target. WhiBal makes a few popular targets, and the XRite Color Checker Passport is also seeing widespread use.

Other: one thing to keep in mind as any subject is photographed from a wide angle is that there will likely be some lens distortion. On a rectangular piece of artwork this would be most noticeable in that the edges of the piece (which are straight lines) might appear to curve a bit as one reaches the edge of the photo. There are a couple possible ways to mitigate this effect:

Use software which can correct the lens distortion. This can be done manually in various programs or automatically with Lightroom's lens correction feature if a profile exists for the lens in use.

Use a tilt-shift lens which allows for the correction to be 'dialed in' at the time of the shoot. Tilt-shift lenses are generally quite expensive and have a learning curve, but they're specifically designed for these sorts of situations.

While a shifting lens can prevent "keystoning" if the camera isn't centered in front of the artwork, it doesn't help barrel distortion at all. The only thing shifting will accomplish is exaggerating the distortion by placing the sensor closer to the edge of the image circle, where distortion is more apparent. If you don't want the image degradation induced by pp correction, get a high-quality rectilinear lens!
– TzariumSep 12 '11 at 15:15

1

Note, though, that macro lenses are generally extremely well-corrected for rectilinearity -- that is, they display a degree of geometric distortion that is only just measurable using high-resolution grid targets. I can't say this about the particular lens under discussion (there are no comprehensive third-party reviews yet), but it does tend to be a sine qua non for a "macro" designation on a prime lens (at least among reputable makers). Keystoning will still be an issue of course, so careful alignment is part of the process.
– user2719Sep 12 '11 at 18:15

Paintings, particularly in oils, tend to be highly reflective (due to protective varnishing), so using "straight" lights, even obliquely, can cause problems. The way around that is to use polarizing filters on both the lights and the camera, with the polarization of the lights at right-angles to the filter on the camera. That eliminates specular reflection from the paint/varnish, and minimizes the texture of the ground (paper/canvas) in the reproduction.

There are specialised lighting fixtures that you can buy that incorporate permanent polarizers, but you can also buy sheets of polarizer film to use with any lighting you have available.

from my experience, glare is the biggest issue and the one most difficult to fix in post.

While everyone says shoot head-on, I find that to be extremely difficult to producing a good image. I use assorted diffuse lighting, no flash. I've tried polarizing filter, but not on both the lens and lighting.

What I find works best is to get even lighting by any method you can. Then I take the shot at a good angle to the painting, looking down. This minimizes glare the most. Distortion and white balance are very easy to fix in photoshop.

Typically I use a canon 10D and now a 7D, most used lens is the 50mm f1.8. Depending on the size of the painting, I may end up taking anywhere from 4 to 16 shots moving across and down the painting in rows. I'll take a "one shot" (whole painting in frame) from a pretty "head-on" angle. I use the one shot as the reference for correcting and placing the other images.

The images of the paintings are used to produce 8 x 10 to 17 x 22 inch prints for sale on assorted sites like Etsy.